Investigators in the electric motor arts have been called upon to significantly expand motor technology from its somewhat static status of many decades. Improved motor performance particularly has been called for in such technical venues as computer design and secondary motorized systems carried by vehicles, for example, in the automotive and aircraft fields. With progress in these fields, classically designed electric motors, for example, utilizing brush-based commutation, have been found to be unacceptable or, at best, marginal performers.
From the time of its early formation, the computer industry has employed brushless D.C. motors for its magnetic memory systems. The electric motors initially utilized for these drives were relatively expensive and incorporated a variety of refinements particularly necessitated with the introduction of rotating disc memory. Over the recent past, the computer industry has called for very low profile motors capable of performing in conjunction with very small disc systems and at substantially elevated speeds.
Petersen, in U.S. Pat. No. 4,745,345, entitled “D.C. Motor with Axially Disposed Working Flux Gap”, issued May 17, 1988, describes a PM D.C. motor of a brushless variety employing a rotor-stator pole architecture wherein the working flux gap is disposed “axially” with the transfer of flux being in parallel with the axis of rotation of the motor. This “axial” architecture further employs the use of field windings which are simply structured, being supported from stator pole core members, which, in turn, are mounted upon a magnetically permeable base. The windings positioned over the stator pole core members advantageously may be developed upon simple bobbins insertable over the upstanding pole core members. Such axial type motors have exhibited excellent dynamic performance and, ideally, may be designed to assume very small and desirably variable configurations.
Petersen in U.S. Pat. No. 4,949,000, entitled “D.C. Motor”, issued Aug. 14, 1990 describes a D.C. motor for computer applications with an axial magnetic architecture wherein the axial forces which are induced by the permanent magnet based rotor are substantially eliminated through the employment of axially polarized rotor magnets in a shear form of flux transfer relationship with the steel core components of the stator poles. The dynamic tangentially directed vector force output (torque) of the resultant motor is highly regular or smooth lending such motor designs to numerous high level technological applications such as computer disc drives which require both design flexibility, volumetric efficiency, low audible noise, and a very smooth torque output.
Petersen et al, in U.S. Pat. No. 4,837,474 entitled “D.C. Motor”, issued Jun. 6, 1989, describes a brushless PM D.C. motor in which the permanent magnets thereof are provided as arcuate segments which rotate about a circular locus of core component defining pole assemblies. The paired permanent magnets are magnetized in a radial polar sense and interact without back iron in radial fashion with three core components of each pole assembly which include a centrally disposed core component extending within a channel between the magnet pairs and to adjacently inwardly and outwardly disposed core components also interacting with the permanent magnet radially disposed surface. With the arrangement, localized rotor balancing is achieved and, additionally, discrete or localized magnetic circuits are developed with respect to the association of each permanent magnet pair with the pole assembly.
Petersen in U.S. Pat. No. 5,659,217, issued Aug. 19, 1997 and entitled “Permanent Magnet D.C. Motor Having Radially-Disposed Working Flux-Gap” describes a PM D.C. brushless motor which is producible at practical cost levels commensurate with the incorporation of the motors into products intended for the consumer marketplace. These motors exhibit a highly desirable heat dissipation characteristic and provide improved torque output in consequence of a relatively high ratio of the radius from the motor axis to its working gap with respect to the corresponding radius to the motors' outer periphery. The torque performance is achieved with the design even though lower cost or, lower energy product permanent magnets may be employed with the motors. See also: Petersen, U.S. Pat. No. 5,874,796, issued Feb. 23, 1999.
Over the years of development of what may be referred to as the Petersen motor technology, greatly improved motor design flexibility has been realized. Designers of a broad variety of motor driven products including household implements and appliances, tools, pumps, fans and the like as well as more complex systems such as disc drives now are afforded an expanded configuration flexibility utilizing the new brushless motor systems. No longer are such designers limited to the essentially “off-the-shelf” motor varieties as listed in the catalogues of motor manufacturers. Now, motor designs may become components of and compliment the product itself in an expanded system design approach.
During the recent past, considerable interest has been manifested by motor designers in the utilization of magnetically “soft” processed ferromagnetic particles in conjunction with pressed powder technology as a substitute for the conventional laminar steel core components of motors. So structured, when utilized as a motor core component, the product can exhibit very low eddy current loss which represents a highly desirable feature, particularly as higher motor speeds and resultant core switching speeds are called for. As a further advantage, for example, in the control of cost, the pressed powder assemblies may be net shaped wherein many intermediate manufacturing steps and quality considerations are avoided. Also, tooling costs associated with this pressed powder fabrication are substantially lower as compared with the corresponding tooling required for typical laminated steel fabrication. The desirable net shaping pressing approach provides a resultant magnetic particle structure that is 3-dimensional magnetically (isotropic) and avoids the difficulties encountered in the somewhat two-dimensional magnetic structure world of laminations. See generally U.S. Pat. No. 5,874,796 (supra) and U.S. Pat. No. 6,441,530.
The above-discussed PM D.C. motors achieve their quite efficient and desirable performance in conjunction with a multiphase-based rotational control. This term “multiphase” is intended to mean at least a three step commutation sequence in conjunction with either a unipolar or bipolar stator coil excitation. Identification of these phases in conjunction with rotor position to derive a necessary controlling sequence of phase transitions traditionally has been carried out with two or more rotor position sensors. By contrast, simple, time domain-based multiphase switching has been considered to be unreliable and impractical since the rotation of the rotor varies in terms of speed under load as well as in consequence of a variety of environmental conditions.
The multiphase motors may be described, for instance, by arbitrarily designating the commutation phase sequence of a three-phase motor as: A, B, and C. During those phases, a three-phase unipolar motor control must determine rotor position information for establishing the transitions from phase A to phase B to phase C to phase A as the sequence continues. Such control has been considered to require three rotor position sensors. The most typical of the position sensors are dual output state Hall devices and optical sensors. Somewhat costlier control also can be achieved with a back EMF circuit monitoring approach which eliminates all physical position sensors.
Still higher efficiencies are achieved with a three-phase bipolar motor wherein such commutation phase sequencing arbitrarily may be designated as calling for transitions from phase AB to phase AC, to phase BC to phase BA, to phase CA, to phase CB, to phase AB as the sequence continues. Here again, a practical control for such motor architecture has been considered to require three rotor position sensors. Four-phase motors with an arbitrarily designated commutation sequence of A, B, C and D are considered to require two rotor position sensors.
While the stator architecture and pressed powder implementation of the above-discussed motors has not only substantially enhanced their practically and has lowered their structural cost, further, quite substantial cost improvements can be realized by limiting the number of bi-state rotor position sensors required for multiphase motors to only one such sensor. In this regard, currently, the multiple sensors must be positioned in substantially spaced apart locations with respect to the rotor or some slave form of sensing structuring. Thus, the significant cost advantages associated with the integration of the positional sensor and the control circuit in a single chip is lost. The resultant cost factor generally precludes the use of efficient multiphase motors with very low cost applications such as electrical circuit cooling fans. However, as the era of electronic-based systems expands, battery-based power limitations are setting the stage for much higher motor efficiency requirements. Those higher efficiencies only are available with multiphase motors. Higher efficiencies for fan motors may be required, for example, for utilization with a rapidly expanding development of laptop computers. The technology long associated with electronic circuit, low load cooling fans has been somewhat static. Usually implemented as D.C. PM devices, the motors have been structured with a single phase or “two-pulse” architecture in order to retain a capability for operation with a single sensor. Such phasing is highly inefficient, the motors necessarily experiencing zero torque based commutation switching.
An implementation of a control system for a multiphase motor which contains only one sensor requires that, from that sensor, effective positional and timing information for carrying out phase commutation transitions in conjunction with reliable performance under load variations extending to those evoking stall phenomena.